It is known in the automotive industry and in other applications to attach a fastener onto a work piece. The fastener, such as a screw, a nut, a stud, etc., is most conventionally attached by resistance electric welding, and the fastener may have small projections that face toward the metal component so that the electric resistance heating is concentrated at these projections in order to facilitate the formation of the weld between the fastener and the metal component. Hence the process is often referred to as projection welding.
It has also known that a pair of work pieces, such as metal sheets, can be joined together by a friction stir rivet process in which a rivet is spun at high speed and advanced into the work pieces, thereby causing the a region of the work pieces surrounding the rivet to be plasticized and then, upon cooling, the rivet is captured within the cooled metal of the work pieces and thereby attaches the work pieces together. This friction stir riveting process is described in U.S. Pat. No. 6,702,535 assigned to the same assignee as this invention and herein incorporated by reference.